feat(server): add trackExposure

[feugy]

🖖 What's in there?

Ports trackExposure() from the @vercel/toolkit prototype to @vercel/analytics/server, so vercel-marketing can report experiment exposures from a canary release.

import { trackExposure } from '@vercel/analytics/server';

await trackExposure({
  experimentId: 'checkout-redesign',
  variantId: 'treatment',
  unitKey: 'user',
  unitValue: user.id,
  userId: user.id,
  props: { plan: 'pro' },
});

It posts to /_vercel/insights/exposure, which api-analytics-ingestion already serves as an alias of /v1/event tagged t: 'exposure'. userId, groupId and props go top-level, matching the fields that service reads and its 256-char truncation.

Server-side only. Browser exposure tracking is a separate ticket.

Landing it meant track and trackExposure shared five near-identical blocks, so the request plumbing now lives in server/request.ts:

• rejectBrowserRuntime() — the typeof window guard
• resolveEndpoint('event' | 'exposure') — adds VERCEL_WEB_ANALYTICS_EXPOSURE_ENDPOINT, and treats VERCEL_WEB_ANALYTICS_ENDPOINT as a base URL for exposures while keeping it verbatim for events
• reportMissingEndpoint() — the dev/prod logging when no endpoint resolves
• dispatch() — request context, header forwarding, the shared envelope, the POST, and the catch-all

track dropped from 84 to 30 lines with no behaviour change: server/index.test.ts is untouched and still green. The server bundle went 8.03 → 7.41 KB.

🤺 How to test?

pnpm -F @vercel/analytics build
pnpm -r test          # 136 unit tests, 18 of them new for trackExposure

src/server/index.test.ts is deliberately unmodified — it is the guardrail proving the track refactor is behaviour-preserving.

🔬 Notes to reviewers

Two intentional behaviour changes:

1. track's browser-guard message said @vercel/web-analytics/server, which is not an importable path. Sharing the guard forced one string, so both now say @vercel/analytics/server. Assertion updated in browser.test.ts.
2. x-va-server: '2' is gone. It sat in a branch guarded by hasHeaders, after an unconditional throw on !hasHeaders — unreachable, so ingestion never received it. Restore the ternary in dispatch() if sending without session headers is ever meant to work.

experimentId over 255 chars is dropped by ingestion with a 202. The SDK does not truncate it, same as the prototype.

[vercel]

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